1. Field of the Invention
The present invention relates to a magnetic material sensing device, a magnetic material sensing method and an image forming apparatus that sense a magnetic material by detecting a signal emitted from the magnetic material which is placed in an alternating magnetic field, and more particularly to a magnetic material sensing device, a magnetic material sensing method and an image forming apparatus having high reliability that, even if a metallic structure is present near a sending coil for sending an alternating magnetic field and a receiving coil for receiving a signal emitted from the magnetic material, can sense surely a signal emitted from the magnetic material by eliminating an influence of the metallic structure.
2. Description of the Related Art
In recent years, there are provided various processes and devices to enhance security for prevention of leakage of confidential information, personal information and the like, prevention of counterfeiting of valuable securities and the like, and theft-prevention of commodities and the like.
For example, for stores and shops where commodities such as various products are sold, there is provided a system capable of preventing commodities from being stolen by fixing a tag to commodities to prevent them from being stolen, and sensing the state of the tag by ON/OFF of 1 bit when a commodity with the tag passes through a gate by a gate-shaped tag sensing apparatus which is installed at the exit so as to judge whether or not the payment for that commodity has been done.
Types of tags used are an RFID (Radio Frequency Identification) tag, a magnetic tag using a magnetic material and the like. And, there are also provided various types of tag sensing devices capable of accurately sensing such tags from a distance near to or far from the tags.
For example, there is a known device for sensing a tag made of a magnetic material. It is configured that a sending coil to which a prescribed exciting signal is applied and a receiving coil which detects a magnetic field are disposed to face each other, and when the tag having the magnetic material moves between the two coils, a change in magnetic flux density caused in the tag by the magnetic field from the sending coil is detected by the receiving coil, it is judged under prescribed conditions that the magnetic field is from the tag, and it is judged that the tag is moving, namely a commodity with the tag is moving, thereby enabling to prevent commodities and the like from being stolen from shops and stores.
Such a device utilizing magnetism detects a signal by the receiving coil when a magnetic material other than the prescribed tag is moving. Therefore, it is important to distinguish the signal from the magnetic field generated by the prescribed tag from the signal according to the magnetic field generated by another magnetic material in order to provide a device with high reliability having a low erroneous discrimination probability and a high precise detection probability.
For example, a highly reliable tag detection method and product monitoring device with a low erroneous discrimination probability and a precise detection accuracy by enabling to extract surely only a signal which is produced due to the magnetic characteristics of a prescribed magnetic member from a tag formed of a magnetic member has been proposed.
The proposed tag detection method and product-monitoring device remove the same-phase component noise with both ends of the receiving coil used as input of a differential amplifier in order to remove exogenous noise generated in the receiving coil from the signal.
And, an S/N ratio is increased by repeating the addition of the detected signal for prescribed times to enhance the accuracy of the detected signal, and an effect of reducing signals other than the target signal detected with irregular timing is obtained by a repeated operation.
The repeated operation is performed with the detection signal digitized, so that ADC (=Analog Digital Converter), memory, CPU (=Central Processing Unit) or the like is required.
But, if a metal structure or a metal not grounded is present near the antenna, multiple reflection occurs between the antenna and the metallic structure, making it difficult to sense the signal emitted from the tag. And, if a metal not grounded is present near the antenna, it is necessary to perform signal processing in order to sense the signal emitted from the tag.